When arc-welding two objects to be welded together along the longitudinal direction of a groove formed therebetween by means of a welding wire such as a consumable welding electrode, welding is usually carried out with a distance of a root gap of the groove of zero, i.e., with the root faces kept in contact with each other. Particularly when conducting automatic arc-welding, it is the usual practice to strictly control the distance of the root gap so that the maximum distance of the root gap is kept at 1 mm. A large distance of the root gap leads to an incomplete penetration and a burn-through. Actually, however, variations in the distance of the root gap in the longitudinal direction of the groove are inevitable because of the limit in accuracy of machining.
It is thus the conventional practice, before welding objects to be welded together along a groove, to modify the distance of the root gap in advance within allowable limits over the entire length of the groove by filling up portions with a large distance of the root gap by a manual welding, for example. In addition, it is necessary to increase a height of the root face so as not to cause a burn-through.
However, such a manual operation to fill up the root gap carried out before the full-scale welding leads to a decreased efficiency of welding operations as a whole. Furthermore, as described above, it is necessary to increase the height of the root face of the groove. As a result, when double-welding the objects to be welded from both the first and second sides thereof, after welding of a first groove formed on the first side and before welding of a second groove formed on the second side, it is necessary to chip the second groove by gouging for example, until the bottom surface of the second groove reaches the weld bead formed in the first groove, in order to ensure a satisfactory penetration.
Under such circumstances, there is a demand for development of an automatic arc-welding method, which ensures a constant depth of penetration, even upon change in the distance of a root gap in the longitudinal direction of the groove. This permits prevention of a burn-through, without increasing a height of the root face, and stabilization of an arc, but such an automatic arc-welding method has not as yet been proposed.